Elastomeric polyurethane coating compositions are of some technical interest. One application area for these coatings would be as a spray-applied fuel bladder for military aircraft. One of the major requirements for this type of coating would be that it swell rapidly when it comes into contact with jet fuel. Further requirements are that it should have good tear strength and a good tensile at relatively high elongation. The material must be able to seal leaks in the fuel tank when the tank is pierced by a projectile or when vibrations of the aircraft cause a rupture of the tank.
Currently, fuel bladders are fabricated in a mold rather than spray applied into the fuel tank. The fabrication process is labor-intensive, hence the cost of the bladders is relatively high. Also, the failure rate of the fabricated bladders at time of installation is rather high. The bladders are collapsed and pulled through a small opening leading to the aircraft's fuel tank in the fuselage and subsequently "sewn" into place. Typical of prior art techniques are those described in U.S. Pat. Nos. 3,509,016, 3,563,846 and 3,698,587.
It would be desirable to replace the fabricated bladder system with a more cost effective one which could be spray applied directly into or onto the fuel tank. Not only would manufacturing and installation costs be lowered, but also the overall fuel capacity would be increased.
Slightly vulcanized (lightly cross-linked) natural gum rubber is presently used for the "swellable" portion of fabricated fuel bladders. This material is tough and swells rapidly in contact with jet fuel, but it is insoluble in coating solvents and thus cannot be applied using conventional coating methods.
Elastomeric, linear polyurethanes are known to swell in hydrocarbon solvents. Most published data reports the extent of polymer swelling after the swollen pieces have reached an optimum (equilibrium) value. In most cases, this does not occur rapidly, but rather over a 2 to 4 day period. This time period is much too long to be practical for the "self-healing fuel tank" application. The polymer must reach an optimum swelling value within a five-minute time frame.
Finally, a variety of different compositions are known which are based on polyisocyanates and hydroxy-terminated polybutadienes (see, e.g., U.S. Pat. Nos. 4,039,489 and 4,396,053).